One kind of actuator typically includes an armature, a spring for urging the armature in one direction, and electromagnetic means energizable to move the armature against the biasing force of the spring. The electromagnetic means may include a coil and a core. By repeatedly energizing the electromagnetic means, the armature can be driven through many cycles of reciprocation each second. Actuators of this type can be used to power various devices, such as positive displacement pumps.
Actuators of this type must necessarily have a short stroke. One problem with these actuators is in properly limiting the length of the stroke. The obvious way to limit the stroke of the armature as it moves toward the coil is with a positive stop. Unfortunately, however, the rapid and repeated contact of the armature and/or the associated structure against a positive stop produces a noise which makes the actuator unsuited for many applications. Moreover, the positive stops tend to wear and, therefore, permit an increase in the length of the stroke with time. Overstroking as a result of the action of the spring in moving the armature could damage the diaphragm.